Superconductivity refers to a state of materials in which the electrical resistance becomes zero when the material is cooled to a sufficiently low temperature, referred to as the critical temperature. One type of superconductor, referred to as a high temperature superconductor (HTS), has a critical temperature in excess of the boiling point of liquid nitrogen of 77° K at atmospheric pressure.
The use of superconductive materials and, in particular, superconducting cables, is advantageous because of the elimination of resistive losses. As a result, superconducting cables are being designed, built and tested for use in industrial applications, such as power transmission systems.
High temperature superconductors may be used to construct superconducting power cables, which are capable of serving very large power requirements at medium voltage ratings. To make a HTS cable operate properly, cryogenic cooling systems with circulating sub-cooled liquid nitrogen are often used to maintain the HTS cable in a superconducting state during normal operation.
Conventionally, cryogenic refrigeration systems are used to cool cold dielectric superconducting cables to a temperature (˜70 K), which may be much lower than the HTS critical temperature (90 K). This may allow the transmission of a higher critical current since the critical current increases dramatically with a decrease in the operating temperature. Such systems may contain a refrigeration unit, a circulating pump and a cooling loop for providing a refrigerant, or coolant, such as liquid nitrogen, at a temperature much lower than the critical temperature. The refrigeration unit is a mechanical refrigeration device that produces cooling power at cryogenic temperature. The coolant, e.g. liquid nitrogen, flows from the refrigeration unit into the cooling loop via a circulating pump. The coolant circulates through the cooling loop extracting heat from the HTS cable, and then returns to the refrigeration system for removal of the heat and circulates back to the cooling loop. The cooling loop could be a tube external to the superconducting cable assembly or the hollow center formed by the superconducting cable former.
One cooling station and one cooling loop are often used for superconducting cable. Unfortunately, as a result of existing cooling system designs, the length of an HTS cable may be limited due to limited pressure head and cooling power produced by a single cooling station. These systems may also require that refrigeration systems be tailored to individual cable projects, which may result in an excessively high refrigeration system cost.